I. Field of the Invention
This invention relates to a detonation assembly for the severing of a wellhead and, in particular, to a connector assembly to supply the firing voltage to the detonator assembly which is selectively engageable to control arming of the explosive and which seals the contact elements to prevent discharge of the power unit prior to engagement for detonation.
II. Description of the Prior Art
Explosives are widely used in drilling operations for the perforation of well casings and the severing of subsea wellheads for their removal. The safe operation of these explosive assemblies is of utmost importance. Potentially armed explosive tools are avoided to ensure detonation only when the device is properly positioned. Most modern explosive devices are triggered by a detonator operated by an electrical charge. Such a charge can be supplied either from the surface or through a separate power unit which is lowered into the hole. The surface detonated devices provide the advantage that detonation can be directly controlled. However, such surface detonated devices require extensive wiring in deep wells and may provide no means of absolutely disarming the explosive in the event of a malfunction. The lowerable units depend upon a battery pack which may become discharged because of the fluid environment.
In one type of previously devised system, after the explosive is positioned, a drop bar is placed in the pipe to travel through the pipe for contact with a firing piece. However, because the explosive is placed in a firing state at the surface before lowering into the well inadvertent firing of the explosive can occur. Moreover, because the drop bar travels in an essentially uncontrolled manner down the pipe it may become lodged in the pipe. In order to correct the problem, the explosive would have to be retrieved from the well which could dislodge the drop bar resulting in detonation at an undesired location, possibly near the surface rig.
Systems which are detonated by a surface operator have proven to be time consuming and expensive. As the drill string is lowered to lower the explosive, successive lengths of pipe are added and the conducting cable is reeled off and attached to the drill pipe. The attachment and retrieval of the cable can be time consuming. Moreover, because the cable is attached to the exterior of the pipe, severing of the cable as the pipe is lowered into the casing is not unknown. Although costly, the system is relatively safe since no power is applied until a specific sequence is conducted by a surface operator.
Other systems have been developed which utilize a lowerable battery or power module to charge the detonator. The explosive charge is positioned within the well. Thereafter, the power unit is lowered into mating engagement with the receiving module and detonation occurs upon contact. Although such devices operate well in dry environments, the highly conductive properties of sea water make such devices unsuitable for subsea operations since the battery pack will likely be discharged prior to engagement. Power units which utilize insulated contacts have been developed but require that the insulation be stripped away to complete the circuit. If the contact does not precisely engage the stripper elements the explosive will not be detonated.